1. Field of the Invention
The present invention relates to electrical contacts, more particularly, to very small compliant electrical contacts with low inductance at high frequencies.
2. The Prior Art
The purpose of an electrical contact is to provide a separable electrical interconnection between two electrical conductors. The characteristic of separability means that the conductors are not interconnected by permanent mechanical means, such as soldering or bonding, but by temporary mechanical means. Consequently, in order to maintain a good mechanical contact in an attempt to minimize detrimental electrical effects of the contact, some form of spring force is used to press the two conductors together. These electrical contacts are called compliant (as in “flexible”) contacts.
Small compliant contacts are necessary for separably interconnecting integrated circuit (IC) devices to whatever electrical device the user desires. A prime example is connecting the IC to a test fixture or sorting equipment used for testing and sorting IC's during manufacture or an Original Equipment Manufacturer (OEM) type connector for connecting an IC to its operating environment such as a CPU in a personal computer, file server or mainframe computer. The compliant contact should be as close to electrically transparent as possible in order to minimize parasitic effects, such as inductance, that alter the signals to and from the IC which could lead to erroneous results.
Compliant contacts provide another advantage in that they can compensate for noncoplanarities of the devices (UUT's) being connected. The conduction points on the UUT's are not exactly coplanar, that is, they are not within the same plane, even between the same conduction point on different UUT's. The compliant contacts deflect by different amounts depending upon the actual position of the conduction point.
Conventional compliant contacts for connecting to UUT's include spring probes, conductive rubber, compliant beam contacts, and bunched up wire called fuzz buttons. Each technology provides the necessary means to overcome the noncoplanarities between the contact points and provides uniform electrical contact throughout a plurality of contacts. Each technology has shortcomings in one characteristic or another and all have high electrical parasitic characteristics. In addition, they are relatively expensive to manufacture.
A typical spring probe consists of at least three or four parts, a hollow barrel with a spring and one or two plungers. The spring is housed in the barrel with the end of the plungers crimped in opposed open ends of the barrel at the ends of the spring. The spring biases the plungers outwardly, thereby providing a spring force to the tip of the plungers. Spring probes can have highly varying degrees of compliance and contact force, and are generally very reliable for making contact many times or for many cycles. Spring probes can accommodate many different conduction interfaces, such as pads, columns, balls, etc. Spring probes, however, have a size problem in that the spring itself cannot be made very small, otherwise consistent spring force from contact to contact cannot be maintained. Thus, spring probes are relatively large, leading to an unacceptably large inductance when used for electrical signals at higher frequencies. Additionally, spring probes are relatively costly since the three components must be manufactured separately and then assembled.
Conductive rubber contacts are made of rubber and silicones of varying types with embedded conductive metal elements. These contact solutions usually are less inductive than spring probes, but have less compliance and are capable of fewer duty cycles than spring probes. The conductive rubber works when the conduction point is elevated off the UUT thus requiring a protruding feature from the UUT or the addition of a third conductive element to the system to act as a protruding member. This third member lessens the contact area for a given contact force and thus increases the force per unit area so that consistent contact can be made. The third element may be a screw machined button which rests on the rubber between the conduction point. This third element can only add inductance to the contact system.
Compliant beam contacts are made of a conductive material formed such that deflection and contact force is attained at one end to the UUT conduction point while the other end remains fixed to the other conductor. In other words, the force is provided by one or more electrically conductive leaf springs. These contacts vary greatly in shape and application. Some compliant beam contacts are small enough to be used effectively with IC's. Some compliant beam contacts use another compliant material, such as rubber, to add to the compliance or contact force to the beam contact point. These later types tend to be smaller than traditional compliant beam contacts and thus have less inductance and are better suited for sorting higher frequency devices.
Fuzz buttons are a relatively old yet simple technology in which a wire is crumpled into a cylindrical shape. The resulting shape looks very much like tiny cylinder made of steel wool. When the cylinder is placed within a hole in a sheet of nonconductive material, it acts like a spring that is continuously electrically shorted. It provides a less inductive electrical path than other contact technologies. Like rubber contacts, the fuzz button is most commonly used with a third element needed to reach inside the hole of the nonconductive sheet to make contact with the fuzz button. This third element increases parasitic inductance, degrading the signals to and from the UUT.
IC packaging technology is evolving toward being smaller, higher frequency (faster), and cheaper, resulting in new requirements for these types of electrical contacts. They need to perform adequately at the lowest cost. One problem with forming electrical connecting elements for such packages is that the dimensions of the contacts are extremely small. As a result, resistance, capacitance, and inductance values are so high as to be almost unacceptable.